Some existing vehicles have exhaust gas aftertreatment systems to reduce the amounts of carbon monoxide, unburned hydrocarbons, and nitrogen oxides (collectively, NOx) that are discharged to the atmosphere in the exhaust from internal combustion engines that power the vehicles. Existing exhaust gas aftertreatment systems may be most effective in treating the exhaust from a warmed-up engine because the catalyst materials have been heated to temperatures (e.g., 200° C. and above) at which the catalyst materials serve to effectively oxidize carbon monoxide and incompletely burned fuel constituents to carbon dioxide and water, and to reduce nitrogen oxides to nitrogen gas. The existing exhaust gas aftertreatment systems have been effective for both gasoline engines operating at or around the stoichiometric air-to-fuel ratio and diesel engines (and other lean-burn engines) operating with excess air (sometimes called “lean burn” engines).
It has been difficult to treat exhaust emissions immediately following a cold engine start, before the exhaust has heated the catalytic converter or converters to the effective temperatures for designated catalytic reactions. Lean-burn engines, such as diesel engines, tend to produce cooler exhaust streams because of the excess air used in the combustion mixtures charged to their cylinders. Untreated cold-start emissions may make-up a significant portion of the total regulated emissions at a tailpipe of a vehicle. Mixed nitrogen oxides in the exhaust of diesel engines have been difficult to reduce. These nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO2); the mixture may be typically referred to as NOx. There is, therefore, a need for better systems for treating the exhaust gas from an engine following a cold-start.